The long range goals of this grant proposal are to elucidate the cellular components involved in the phenomenon known as transcription- dependent DNA repair, which is characterized by the preferentially biased clearance of DNA damage from the transcribed strand of actively expressed genetic loci. This results in significant modulation of the biological consequences of chemical-induced, modified bases in DNA, including mutagenicity, toxicity, and carcinogenicity. Ethylating agents are one class of environmentally relevant compounds that damage DNA, yielding adducts that are cleared in a preferential fashion from transcribed genomic domains. The hypothesis of this application is that transcription-dependent clearance of N-ethylpurines exists in DNA repair proficient mammalian cells but is absent in cells lacking functional nucleotide excision repair or mismatch repair. The specific aims designed to test this are: (1) The repair of N-ethylpurines will be assessed in active genetic loci in mammalian cells that are proficient in DNA repair; (2) the gene-specific removal of N-ethylpurines will be evaluated in cell lines derived from patients with the nucleotide excision repair deficiency disease xeroderma pigmentosum; (3) the clearance of these adducts from cells originating from patients having Cockayne's syndrome will be determined; (4) N-ethylpurine clearance will be measured in human cells deficient in functional mismatch repair; and (5) the gene-specific removal of these adducts will be assessed in murine cells lacking the base excision repair pathway. The results obtained from these studies will begin to define the precise gene products necessary for the transcription-dependent clearance of lesions formed in DNA, and the data will be compared to results for the transcription-dependent clearance of adducts. These findings will add an important contribution to our understanding of the mechanism underlying transcription-dependent DNA repair and DNA repair in general.